1. Field of the Invention
The invention relates to a power assist device, and more particularly to a power assist device that guides an advancing direction when a work is conveyed and to a control method for the power assist device.
2. Description of the Related Art
Within the framework of the related technology, robots called power assist devices have been used in production plants to reduce the operator's labor and increase operability. As a technique relating to power assist devices, there is a conventional technique of driving a motor of an arm supporting a work so that, for example, when the work protrudes from a set area, which has been set in advance, during conveying, a return force received from a virtual wall called “an invisible wall” is provided to the operator (for example, see Japanese Patent Application Publication No. 2005-28492 (JP-A-2005-28492)).
As an application example of a power assist device in a production plant, a power, assist device in which an operation member operated by an operator is installed at a distal end of a robot arm performing the assist and a force assisting the operator's operation is generated in the robot arm is used for highly accurate work alignment and operations requiring accuracy of advancing direction during fitting, for example, when a window glass is fitted into an automobile body.
However, with the technology described in JP-A-2005-28492, the work trajectory that serves as a reference during conveying cannot be easily changed or modified. As a result, for example, where such a technology is applied to the above-described operation of fitting a window glass in an automobile body, the following drawbacks are encountered when the body and window glass are displaced even if slightly from a teaching set area: (1) an up-stopper of the window glass is not appropriately inserted into an engagement hole of the body and hits the body, and (2) the glass hits a trunk or the body when the window glass is inserted in the gap between the trunk and the body. Yet another drawback is that when the teaching set area is changed, time is required for changing the set area.
Furthermore, the following specific problems arise when the technology described in JP-A-2005-28492 is applied to an operation in which window glass covered with an urethane adhesive (reference numeral 12 in FIG. 4) is positioned in the direction other than the vertical direction at a certain height above the body so as to prevent the adhesive from adhering to the body, and the window glass is then lowered, while maintaining the position thereof: (1) high accuracy of conveying direction is required because the gap between the trunk and body and the orientation of the stopper for inserting into the holes of the body are determined according to the work structure; (2) when a force is applied to move an operation handle, the force is also applied in the direction in which the work is not intended to be moved, the work moves in this direction, the position thereof is displaced, and the insertion orientation is changed; (3) because of a spread in body positions between the fitting cycles, the set teach position has to be changed, and when the teaching set area is changed, the conveying mode and a mode of changing the set area have to be switched.
Furthermore, where an operation of fitting a window glass into an automobile body is performed without applying the technology described in JP-A-2005-28492, that is, where an operation of fitting the window glass 2 into the body 100 is performed by an operator, while the window glass is being moved by the power assist device of related technology, as shown in FIG. 10, when the operator performs the operation of the robot arm providing the assist with an operation member, the robot arm can be moved in the same manner in any direction, and even if the front and rear positions of the window glass 2 are aligned in a space above the automobile body 100 in this state, the window glass can shift forward or rearward as it is being lowered in the direction shown by a dot-line arrow in FIG. 10. Accordingly, a means is desired for lowering the window glass straight down with good stability, while maintaining the alignment of the upper position.